1. Field of the Invention
The present invention relates to a seat back structure of an automotive seat, and in particular is directed to a back support structure of the seat back.
2. Description of Prior Art
The recent years have seen a certain improvement in a seat technology field, especially many seat technical people having made an attempt to improve a support structure of the seat. The same concern has also been drawn to the back support structure of a seat back forming one of the seat, and for instance, it has been seen that the provision of a synthetic resin back support plate within the seat back is effective in supporting the upper back part and lumbar part of an occupant on the seat at a pleasant touch. Such resin back support plate is made of such relatively hard yet somewhat elastic synthetic resin as P.V.C., polyethylen, resin, polystyrene resin, or polymide; resin, which should be provided a proper flexture property by adding a certain amount of plasticizer therein.
As an example of the seat back of this kind, reference is made to FIGS. 1 through 3, in which FIG. 1 shows a general structure of a seat back (3) in an automotive seat (1). Designation (2) denotes a seat cushion connected with the seat back. In FIG. 1, in general, a back support plate (P) of a synthetic resin is provided within the seat back (3), supported elastically by a seat back frame (F), and covered with a cushion layer (32) and top cover layer (31), as shown.
FIGS. 2 and 3 show a conventional seat back structure. Designation (16) shows a seat back frame which is to be provided within the foregoing seat back (3). On the lateral frame sections of the seat back frame (16), are fixed respectively two side auxiliary frames (18). Each of the side auxiliary frames (18) is formed with three securing lugs (19) projecting inwardly of the frame (16). Generally centrally of the seat back frame (16), is disposed a back support plate (14) made of such synthetic resin material as mentioned above. The back support plate (14) is formed with plurality of small holes (15), as shown, which serve to render more elastic the back support plate (14) for the purpose of giving a most comfortable support touch to the back part of the occupant. The back support plate (4) is resiliently supported within the frame (16) by means of an upper set of tension springs (17a) (17a), an intermediate set of tension springs (17b) (17b), and a lower set of tension springs (17c) (17c), all of those springs are extended between the respective securing lugs (19) and lateral edge of the support plate (14). In this regard, each of the springs (17a) (17d) (17c) is fitted at its one hook end part in the respective securing holes (19a) of the securing lugs (19) and at its other hook end part in the respective most lateral-side holes (15) of the support plate (14), as best seen from FIG. 3. The back support plate (14) is formed in a little wavy contour conforming to that of occupant's standard back, and the lower set of tension springs (17c)(17c) are of a higher resiliency than other ones (17a)(17b), with a view to improving a back support comfort of the seat back.
However, an upleasing objection is still raised from the occupant with regard to the support touch of the abovementioned prior art: As the back support plate (14) is resiliently supported only by the springs (17a)(17b)(17c), when a load (F) is applied to the support plate (14), as in FIG. 3, initially, the support plate (14) is not given the direct resilient repercussive force of the springs (17a)(14b)(17c), by reason of the fact that, in such case, the springs (17a)(17b)(17c) are simply fitted in both securing holes (19a) of the securing lugs (19) and holes of the support plate (14) and thus, rotated downwardly from the solid line to the phantom line about their respective securing holes (19a), without being applied the direct downward load (F). As a result, the occupant feels a sudden backward movement in the seat back and uneasy with such insufficient support touch.